Well my friend got a new Mac and hes young so he wants to play downloaded games like counterstrike, maplestory, rakion, gunbound etc. These games are all Legit for sure. He says when he tries to open one of those games like Maplestory, a pop-up came up which said that the game could not be opened in DOS mode or something. I'm trying to help him play downloadable games as said above. He says he has no idea what version it is. It's like iMac or something. What ways are there to play downloaded games?

The problem is those games require Microsoft Windows to run. They are designed for a PC not a Mac. You can make a Mac dual boot with windows but he will need to buy a copy of Windows to be able to do this.

Post back if you would like more info on how to do this.

Make sure that the games say that they will run on Mac.
Some that run on mac are:

Diablo II
World Of Warcraft
Heroes V of might and magic

Most games don't run on Mac, but some very good ones do.